A SIMPLE-MODEL FOR NONEQUILIBRIUM FLUCTUATIONS IN A FLUID

in equilibrium systems, thermal fluctuations are correlated in time (f luctuation-dissipation theorem) yet their instantaneous values are unc orrelated in space over macroscopic distances. Out of equilibrium, lon g range spatial correlations are predicted by theory and have been obs erved in both laboratory experiments and computer simulations. In this paper, we present a simple model, called the train model, that illust rates this phenomenon. The theoretical analysis of the model and its c onnection with fluctuating hydrodynamics are outlined. (C) 1996 Americ an Association of Physics Teachers.